The present disclosure generally relates to acidizing subterranean formations, and, more specifically, to methods for acidizing subterranean formations in the presence of a chelating agent that is initially unable to complex a metal ion.
Treatment fluids can be used in a variety of subterranean treatment operations. Such treatment operations can include, without limitation, drilling operations, stimulation operations, production operations, remediation operations, sand control treatments, and the like. As used herein, the terms “treat,” “treatment,” “treating,” and grammatical equivalents thereof refer to any subterranean operation that uses a fluid in conjunction with achieving a desired function and/or for a desired purpose. Use of these terms does not imply any particular action by the treatment fluid or a component thereof, unless otherwise specified herein. More specific examples of illustrative treatment operations can include drilling operations, fracturing operations, gravel packing operations, acidizing operations, scale dissolution and removal operations, sand control operations, consolidation operations, and the like.
Acidizing operations may be used to stimulate a subterranean formation to increase production of a hydrocarbon resource therefrom. Introduction of the acidizing fluid to the subterranean formation may take place at matrix flow rates without fracturing of the formation matrix, or at higher injection rates and pressures to fracture the formation (i.e., an acid-fracturing operation). During an acidizing operation, an acid-soluble material in the subterranean formation can be dissolved by one or more acids to expand existing flow pathways in the subterranean formation, to create new flow pathways in the subterranean formation, and/or to remove acid-soluble precipitation damage in the subterranean formation. The acid-soluble material being dissolved by the acid(s) can be part of or formed from the native formation matrix or can have been deliberately introduced into the subterranean formation in conjunction with a stimulation or like treatment operation (e.g., proppant or gravel particulates). Illustrative substances within the native formation matrix that may be dissolved by an acid include, but are not limited to, carbonates, silicates and aluminosilicates. Other substances can also be dissolved during the course of performing an acidizing operation, and the foregoing substances should not be considered to limit the scope of substances that may undergo acidization.
Carbonate formations can contain minerals that comprise a carbonate anion (e.g., calcite (calcium carbonate) and dolomite (calcium magnesium carbonate)). When acidizing a carbonate formation, the acidity of the treatment fluid alone can be sufficient to solubilize the carbonate material by decomposing the carbonate anion to carbon dioxide and leeching a metal ion into the treatment fluid. Both mineral acids (e.g., hydrochloric acid) and organic acids (e.g., acetic and formic acids) can be used to treat a carbonate formation, often with similar degrees of success.
Siliceous formations can include minerals such as, for example, zeolites, clays, and feldspars. As used herein, the term “siliceous” refers to a substance having the characteristics of silica, including silicates and/or aluminosilicates. Dissolution of siliceous materials through acidization is thought to be considerably different than acidizing carbonate materials, since the mineral and organic acids that can be effective for acidizing carbonate materials may have little effect on a siliceous materials. In contrast, hydrofluoric acid, another mineral acid, can react very readily with siliceous materials to promote their dissolution. Oftentimes, a mineral acid or an organic acid can be used in conjunction with hydrofluoric acid to maintain a low pH state as the hydrofluoric acid becomes spent during dissolution of a siliceous material. In addition to siliceous materials, many types of siliceous formations can also contain varying amounts of carbonate materials. Most sandstone formations, for example, contain about 40% to about 98% sand quartz particles (i.e., silica), bonded together by various amounts of cementing materials, which may be siliceous in nature (e.g., aluminosilicates or other silicates) or non-siliceous in nature (e.g., carbonates, such as calcite).
In some instances, it can be desirable to remove a carbonate material from a siliceous formation prior to acidizing the siliceous material therein. A leading reason to remove a carbonate material separately from a siliceous material is that calcium ions liberated from the carbonate material can react readily with fluoride ions to form highly insoluble calcium fluoride, which can be more damaging to the subterranean formation than if the acidizing operation had not been performed in the first place. Another approach that can be used in this regard is to employ chelating agents that effectively sequester the liberated calcium ions, such that they are substantially unable to undergo a further reaction to produce calcium fluoride. Chelating agents can also be used with similar benefits in conjunction with acidizing a carbonate formation. Normally, when acidizing a subterranean formation of any type in the presence of a chelating agent, the pH of the acidizing fluid is maintained in a range where the chelating agent has a pair of free electrons that it can donate to form a metal-ligand bond. Otherwise, the chelating agent can be ineffective for undergoing metal ion complexation. For example, in the case of carboxylic acid-containing chelating agents, the carboxylic acid group normally needs to be deprotonated in order for effective complexation of a metal ion to take place. Certain metal ions can also be more effectively complexed within some pH ranges than in others.
When acidizing a carbonate formation with a mineral acid or an organic acid, and to a somewhat lesser extent a siliceous formation also containing a carbonate material therein, it can be desirable to increase the permeability of the formation through generation of wormholes in the formation matrix without increasing the skin value of the formation. As used herein, the term “skin value” refers to a quantitative measure of damage that is present in a subterranean formation. As used herein, the term “wormhole” refers to a channel generated in the matrix of a subterranean formation through dissolution of a material therein, particularly a carbonate material. During acidizing operations conducted at matrix flow rates (i.e., below the fracture gradient of a subterranean formation), wormhole generation can be desirable in order to increase the permeability of the subterranean formation. In many instances, however, the rapid reaction of acids with carbonate materials can result in bulk erosion of the formation matrix, rather than the desired generation of wormholes, and increased permeability may not be realized. In acid-fracturing operations, wormhole generation can sometimes be less desirable, since wormholes can divert an acid from a desired location and decrease the amount of acid etching that occurs in a generated fracture.
As noted above, bulk erosion of the formation matrix can be problematic during acidizing operations due to the rapid reaction of acids with carbonate materials. One technique that may be used to lower these reaction rates is to decrease the acid concentration, but this approach can result in the acidizing fluid becoming spent too quickly. Another approach that may be used to decrease reaction rates is to viscosify the acidizing fluid. Suitable viscosifed acidizing fluids that may be used in this regard include, for example, acid-stable emulsions, gelled fluids based on acrylamide polymers, viscoelastic fluids based on sulfonated acrylamide polymers, and viscosified fluids containing non-polymeric viscoelastic surfactants. However, these approaches can considerably add to the cost and complexity needed to effectively conduct an acidizing operation. Moreover, the viscosifying agents have the potential to damage the subterranean formation.